The invention relates to a clean metal nucleated casting article, associated methods, and systems for forming the article.
Metals, such as iron-(Fe), nickel-(Ni), titanium-(Ti), and cobalt-(Co) based alloys, are often used in turbine component applications, in which fine-grained microstructures, homogeneity, and essentially defect-free compositions are desired. Problems in superalloy castings and ingots are undesirable as the costs associated with superalloy formation are high, and results of these problems, especially in ingots formed into turbine components are undesirable. Conventional systems for producing castings have attempted to reduce the amount of impurities, contaminants, and other constituents, which may produce undesirable consequences in an article made from the casting. However, the processing and refining of relatively large bodies of metal, such as superalloys, is often accompanied by problems in achieving homogeneous, defect-free structure. These problems are believed to be due, at least in part, to the bulky volume of the metal body.
One such problem that often arises in superalloys comprises controlling the grain size and other microstructure of the refined metals. Typically, refining processing involves multiple steps, such as sequential heating and melting, forming, cooling, and reheating of the large bodies of metal because the volume of the metal being refined is generally of at least about 5,000 and can be greater than about 35,000 pounds. Further, problems of alloy or ingredient segregation also occur as processing is performed on large bodies of metal. Often, a lengthy and expensive sequence of processing steps is selected to overcome the above-mentioned difficulties, which arise through the use of bulk processing and refining operations of metals.
A known such sequence used in industry, involves vacuum induction melting; followed by electroslag refining (such as disclosed in U.S. Pat. Nos. 5,160,532; 5,310,165; 5,325,906; 5,332,197; 5,348,566; 5,366,206; 5,472,177; 5,480,097; 5,769,151; 5,809,057; and 5,810,066, all of which are assigned to the Assignee of the instant invention); followed, in turn, by vacuum arc refining (VAR) and followed, again in turn, by mechanical working through forging and drawing to achieve a fine microstructure. While the metal produced by such a sequence is highly useful and the metal product itself is quite valuable, the processing is quite expensive and time-consuming. Further, the yield from such a sequence can be low, which results in increased costs.
Vacuum induction melting of scrap metal into a large body of metal, such as at least 20,000 pounds, can be useful for scrap material recovery. The scrap is processed by vacuum induction melting steps to form a large ingot product. This type of large ingot product has considerably more value than the scrap, however the large ingot product usually contains one or more defects, such as but not limited to, voids, cracks, oxide inclusions, and macrosegregation. The scrap metal recovery into an ingot is often the first step in an expensive, time-consuming metal-refining process. Subsequent processing steps are used to remedy defects generated during the prior metal processing steps. For example, after the scrap metal is formed into a large ingot, the ingot is often processed by electroslag refining to remove impurities, contaminants, oxides, sulfides, and other undesirable constituents. The electroslag refining process product usually contains lower concentrations of impurities.
Problems may also arise during some electroslag refining processing operations. For example, a conventional electroslag refining process typically uses a refining vessel that contains a slag refining layer floating on a layer of molten refined metal. An ingot of unrefined metal is generally used as a consumable electrode and is lowered into the vessel to make contact with the molten electroslag layer. An electric current is passed through the slag layer to the ingot and causes surface melting at the interface between the ingot and the slag layer. As the ingot is melted, oxide inclusions or impurities are exposed to the slag and removed at the contact point between the ingot and the slag. Droplets of refined metal are formed, and these droplets pass through the slag and are collected in a pool of molten refined metal beneath the slag.
The above-discussed electroslag refining apparatus may be dependent on a relationship between the individual process parameters, such as, but not limited to, an intensity of the refined current, specific heat input, and melting rate. This relationship involves undesirable interdependence between the rate of electroslag refining of the metal, metal ingot temperature, and rate at which the refined molten metal is cooled, all of which may result in poor metallurgical structure in the resultant casting.
Another problem that may be associated with conventional electroslag refining processing comprises the formation of a relatively deep metal pool in an electroslag crucible. A deep melt pool causes a varied degree of ingredient macrosegregation in the metal that leads to a less desirable microstructure, such as a microstructure that is not a fine-grained microstructure, or segregation of the elemental species so as to form an inhomogeneous structure. A subsequent processing operation has been proposed in combination with the electroslag refining process to overcome this deep melt pool problem. This subsequent processing may be vacuum arc remelting (VAR). Vacuum arc remelting is initiated when an ingot is processed by vacuum arc steps to produce a relatively shallow melt pool, whereby an improved microstructure, which may also possess a lower hydrogen content, is produced. Following the vacuum arc refining process, the resulting ingot is then mechanically worked to yield a metal stock having a desirable fine-grained microstructure. Such mechanical working may involve a combination of steps of forging and drawing. This thermo-mechanical processing requires large, expensive equipment, as well as costly amounts of energy input.
An attempt to provide a desirable casting microstructure has been proposed in U.S. Pat. No. 5,381,847, in which a vertical casting process attempts to control grain microstructure by controlling dendritic growth. The process may be able to provide a useable microstructure for some applications, however, the vertical casting process does not control the source metal contents, including but not limited to impurities, oxides, and other undesirable constituents. The uncontrolled source metal content may adversely impact a casting's microstructure and characteristics.
Therefore, a need exists to provide metal casting process that produces a casting with a relatively homogeneous, fine-grained microstructure, in which the process does not rely upon multiple processing steps, and is supplied with a clean metal source. Further, a need exists to provide a metal casting system that produces a casting with a relatively homogeneous, fine-grained microstructure. Further, a need exists to provide a metal casting process and system that produces a casting that is essentially free of oxides, for turbine component applications.